METALLURGY OF IKON AND STEEL.
theory is in accord with the purification of pig-iron by the addition of spiegel.
All the components enumerated are fixed and determined agents in the transactions. Manganese is sometimes reduced from the slag by the .carbon of the bath, and a certain percentage may remain un-oxidized in the metal, but aside from this the oxides of aluminum, silicon and manganese exist in the slag in just the quantities that were added with the stock; but there are three other constituents— iron oxide, phosphoric acid, and sulphur—whose presence in the slag is determined by the conditions of manipulation and by the proportions of other constituents. Iron oxide is always present, the exact amount depending upon the reducing power of the carbon of the bath. It matters not whether ore is added before melting, after melting, or not at all; there is a certain content of FeO which is demanded by existing conditions, and .that certain content will be present. An exception must be made in the case of ore added after the carbon is nearly eliminated, but aside from this there will be just as much iron oxide lost in the slag when no ore is used as when it has been added in proper quantity, and, therefore, all the ore is clear gain.
The presence of iron oxide in either acid or basic slag is an anomaly, for in an acid charge the oxidation of the silicon and manganese would be sufficient to produce a slag without other aid. Nevertheless, there is a force at work in an acid furnace which is constantly creating a slag with about 50 per cent. Si02 and 45 per cent. FeO-|-MnO. If more FeO is added, the carbon of the metal seizes the oxygen and sets free metallic iron, but the same powerful action which so quickly accomplishes- the destruction of this excess is not able to pass much below the limit, even by exposure for hours, without any addition of ore. There is an automatic adjustment to a fixed status which is one of the most wonderful phenomena of chemical physics. The only explanation I can offer is that forces work along the lines of least resistance, so that a slag will seek to combine with anything that promotes fusibility. If given the op^ portunity, a silicious slag absorbs either bases or silica, but preferably bases, and particularly those which impart the greatest fluidity. This action tends to continue indefinitely, and in an acid furnace, if the heat is not tapped after the carbon is burned, the formation of iron oxide will go on with great rapidity, and the fluidity of the